docs: def unique_slug_required(form, slug):
      """Enforce a unique slug accross all pages and websistes."""
  
      if hasattr(form, 'instance') and form.instance.id:
          if Content.objects.exclude(page=form.instance).filter(
              body=slug, type="slug").count():
              raise forms.ValidationError(error_dict['another_page_error'])
      elif Content.objects.filter(body=slug, type="slug").count():
          raise forms.ValidationError(error_dict['another_page_error'])
      return slug